In conventional optical networks, it is known to use wavelength division multiplexing to transmit multiple data channels along a single fiber. Such networks typically have multiple nodes connected in point to point, ring or mesh configurations. Optical add drop modules (OADM) are used at intermediate nodes in such networks to add or drop wavelengths, and pass through the remaining wavelengths. It has long been desired to provide reconfigurable OADMs to enable remote controlled selection of which wavelengths to add or drop at a particular node, or to control routing of wavelengths between nodes, for example for protection switching purposes. 2×2 optical switches may be provided for switching the multiplexed optical signals from one of two fibers to an OADM.
It is known from U.S. Pat. No. 6,084,694 (Milton) to have the wavelengths organized into bands, an interface at each node for dropping a band associated therewith, adding a band carrying traffic for another node, and passively forwarding other bands. This enables the bands to be passed directly between any pair of nodes in said network without the active intervention of any intervening node. The use of bands as distinct from discrete wavelengths allows the filter specifications to be relaxed in the area of sideband roll-off slope since there are cascaded filters involved at each node. A primary (or band) filter discriminates a band of wavelengths. Further sub-division into specific channels is done with a narrow width filter(s) after the band filter. The use of a multi-level filtering approach is more efficient in optical power terms than other arrangements for ring networks. This is due to the fact that the band filter is the primary filter element that is repeated around the ring. As nodes are added to the ring, the attenuation loss due to the band filter element does not rise as fast as the case where individual wavelengths are filtered out at a node with the residual band being passed on. After conversion to electrical form, a digital cross connect can enable reconfiguration of which data is transmitted on which wavelength.
Another example of a banded add drop filter is shown in U.S. Pat. No. 6,243,179 (Thompson). To reconfigure the filter to pass through a given one of the bands, fiber jumper leads can be used. In these patents the term “band” is used to indicate a subset of the wavelengths carried by the fiber, or amplified by a single optical amplifier. As this terminology is inconsistent with the terms C or L band, which are commonly used to indicate all the wavelengths carried or amplified, in contrast, in the remainder of this document, the term “band” will be used to indicate all the wavelengths, and “sub band” will indicate a subset, such as a half band, or one eighth of the band for example.
It is known to provide coarse demux on one card, to output half-band or sub band signals and wavelength or channel demux on a separate card to output individual wavelengths. The half bands or subbands are coupled between the cards by fiber patch cords. To reconfigure which wavelengths or bands are added or dropped, the patch cords, can be plugged or unplugged manually. This leaves problems of cost and delay involved in sending a worker to a remote site of the node, as well as the space or planning required to allocate space in shelves in the system, required for many such cards in a typical high density approach to the system.
As traffic over a network grows, an operator may wish a mid-life upgrade of an express node (which merely amplifies or routes the entire band or in some cases half bands with no adding or dropping of wavelengths), to an add/drop node capable of adding/dropping individual wavelengths. This will require adding additional subband demux cards, which will incur additional cost and footprint to the node as well as the operation of unplugging half band demux cards and reconnecting them to the new cards. Thus it is difficult or expensive to allow or plan for such upgrade at the time of initial installation, and may cause an undesirable interruption to service of bands being passed through. Especially in future systems, as there is a possibility for the need to upgrade to provide more amplification of coarse bands, for higher bit rates, which would involve additional planning and space for the scalable nodes, which is often impractical or expensive.
It is also known from U.S. Pat. No. 6,020,986 to provide a reconfigurable OADM which can be reconfigured without a halt in service. This uses an array of programmable fiber Bragg gratings, one for each wavelength, so that individual wavelengths can be added or dropped.
It is also known to have arrays of optical switches for switching individual wavelengths, such as well known MEMS (Micro Electro Mechanical Switches) devices coupled to a wavelength demultiplexer.